1. Field of the Invention
The present invention relates to a power transmission mechanism for a front and rear-wheel drive vehicle in which one of pairs of front and rear drive. Wheels is driven by an engine whereas the other pair is driven by an electric motor.
2. Description of the Related Art
Known as a conventional power transmission mechanism of this type is a power transmission mechanism which is disclosed in, for example, JP-A-9-315164. This power transmission mechanism is applied to a hybrid vehicle in which front wheels are driven by an engine whereas rear wheels are driven by an electric motor. The power transmission mechanism comprises a viscous coupling provided between the electric motor for driving the rear wheels and the rear wheels, and two clutches for providing engagement and disengagement between the viscous coupling and the left and right rear wheels, respectively. In this power transmission mechanism, when driving the rear wheels, the power of the electric motor is transmitted to the left and right rear wheels via the viscous coupling and the left and right clutches. On the contrary, at other times than of driving the rear wheels, the two clutches disengage the viscous coupling from the left and right rear wheels, whereby the electric motor is prevented from constituting a rotational resistance while running, to improve the fuel economy. In addition, the viscous coupling and the electric motor are coupled together via a speed reduction mechanism which is constituted by two bevel gears which mesh with each other.
In a hybrid vehicle as described above, an electric motor is installed in addition to an engine, and therefore it is desirable to have a power transmission mechanism made as small as possible. In the above conventional power transmission mechanism, however, the speed reduction is implemented only in one stage by the two bevel gears except for the speed reduction implemented by the viscous coupling. Therefore, when trying to ensure a desired running performance, the motor cannot be made smaller but a larger motor has to be used. Thus, it becomes difficult to have a miniaturized power transmission mechanism. In other words, a miniaturized electric motor reduces the power that is to be transmitted to the rear wheels in accordance with the degree of miniaturization thereof, leading to the deterioration of the running performance. In order to compensate for the decrease in running performance, there is considered the increasing of the reduction ratio of the speed reduction mechanism. In such a case, however, the bevel gear on the downstream side or on the viscous coupling side becomes larger, and this causes an enlargement of the speed reduction mechanism in contradiction to the desire, making it difficult to miniaturize the power transmission mechanism.
The invention was made in view of the above situation, and an object thereof is to provide a power transmission mechanism for a front and rear-wheel drive vehicle which can realize the miniaturization of the power transmission mechanism while maintaining a desired running performance, whereby the fuel economy is improved.
With a view to attaining the object, according to a first aspect of the invention, there is provided a power transmission mechanism 1 for a front and rear-wheel drive vehicle 2 in which one of pairs of front and rear drive wheels (for example, front wheels 4, 4 in an embodiment (hereinafter, the same in this first aspect) ) is driven by an engine 3 whereas the other pair (rear wheels 6, 6) is driven by an electric motor 5 for transmitting the power from the electric motor 5 to the other pair of drive wheels (rear wheels 6, 6), the power transmission mechanism comprising an output shaft 11 adapted to rotate together with the electric motor 5, a rotatable middle shaft 12 which is parallel with the output shaft 11, a drive shaft 13 which is parallel with the output shaft 11 and is adapted to rotate together with the other pair of drive wheels (rear wheels 6, 6), a middle shaft speed reduction mechanism (a first pair of reduction gears 14b) having an output shaft speed reduction member (a reduction gear 14a) provided on the output shaft 11 and a first middle shaft speed reduction member (a reduction gear 14b) provided on the middle shaft 12 and adapted to reduce the speed of the middle shaft 12 relative to the output shaft 11 through coupling of the output shaft speed reduction member (the reduction gear 14a) and the first middle shaft speed reduction member (the reduction gear 14b), a drive shaft speed reduction mechanism (a second pair of reduction gears 15) having a second middle shaft speed reduction member (a reduction gear 15a) provided on the middle shaft 12 and a drive shaft speed reduction member (a reduction gear 15b) provided on the drive shaft 13 and adapted to reduce the speed of the drive shaft 13 relative to the middle shaft 12 through coupling of the second middle shaft speed reduction member (the reduction gear 15a) and the drive shaft speed reduction member (the reduction gear 15b), and a differential (a rear differential 20) disposed closer to the electric motor 5 in an axial direction of the drive shaft 13 than the drive shaft speed reduction mechanism (the second pairs of reduction gears 15) for coupling the drive shaft speed reduction member (the reduction gear 15b) and the drive shaft 13, 13.
According to the power transmission mechanism for a front and rear-wheel drive vehicle, the power of the electric motor is reduced in speed in one stage by the middle shaft speed reduction mechanism between the output shaft and the middle shaft and is then reduced in another stage by the drive shaft speed reduction mechanism between the middle shaft and the drive shaft, whereby the power is transmitted to the drive shaft and the other pair of drive wheels which are made integral with the drive shaft via the differential 20 after it has been reduced in speed in two stages. Thus, since the power of the electric motor is transmitted to the other pair of drive wheels in a state in which the power is reduced in speed in two stages by the two speed reduction mechanisms, a larger reduction ratio can be obtained without enlarging the speed reduction mechanism when compared to the conventional power transmission mechanism in which a set of gears is used to reduce in speed the power of the electric motor. As a result, the electric motor can be made smaller while maintaining the running performance. In addition, since the two speed reduction mechanisms are constituted by the three rotating shafts (the output shaft, the middle shaft and the drive shaft) which are parallel with one another and the four speed reduction members in total which are provided on these rotating shafts, the axial dimensions of the respective rotating shafts can be. Controlled.
Consequently, being different from the conventional power transmission mechanism, with the power transmission mechanism according to the first aspect of the invention, the power transmission can be miniaturized, and as a result the fuel economy can be improved. Furthermore, since the differential is disposed closer to the electric motor than the drive shaft speed reduction mechanism in the axial direction of the drive shaft, the electric motor and the differential which are heavier in weight of the constituent components of the power transmission mechanism can be disposed centrally between the left and right drive wheels with a dynamically good balance, and the lengths of the two drive shafts for connecting the differential with the left and right drive wheels can be made substantially equal to each other, whereby the left and right drive wheels can be driven with an ideally good balance.
According to a second aspect of the invention, there is provided a power transmission mechanism 1 for a front and rear-wheel drive vehicle 2 as set forth in the first aspect of the invention, further comprising a clutch 17 provided on the middle shaft 12 for providing engagement and disengagement between the first middle shaft speed reduction member (the reduction gear 14b) and the second. Middle shaft speed reduction member (the reduction bear 15a).
According to the power transmission mechanism for a front and rear-wheel drive vehicle, since the clutch is provided on the middle shaft, the engagement of the clutch can be implemented in a state in which the differential rotation between clutch members is small when compared to a case where the clutch is provided on the output shaft, and therefore the wear of the clutch members and the generation of heat therein can be suppressed, whereby the durability of the clutch can be improved. On the other hand, when compared with a case where the clutch is provided on the drive shaft, since the engagement of the clutch can be implemented in a state in which torque to be transmitted by the clutch is small due to the fact that speed reduction until then is implemented in one stage, the capacity of the clutch can be made small. Consequently, both the improvement in the durability of the clutch and reduction in the capacity thereof can be attained with good balance.
According to a third aspect of the invention, there is provided a power transmission mechanism 1 for a front and rear-wheel drive vehicle 2 as set forth in the second aspect of the invention, wherein the clutch 17 is provided on the middle shaft 12 and is disposed on an opposite side to the electric motor 5 relative to the first middle shaft speed reduction member (the reduction gear 14b) and the second middle shaft speed reduction member (the reduction gear 15a) in an axial direction of the middle shaft 12.
According to the power transmission mechanism for a front and rear-wheel drive vehicle, since the clutch is disposed on the opposite side to the electric motor relative to the first and second middle shaft speed reduction members in the axial direction of the middle shaft, not only can a space for installing a drive mechanism for driving the clutch be easily secured, but also work can easily be carried out for engaging this drive mechanism with a power source.
According to a fourth aspect of the invention, there is provided a power transmission mechanism 1 for a front and rear- wheel drive vehicle 1 as set forth in any of the first to third aspects of the invention, further comprising first and second casing members 31, 32 for accommodating the output shaft 11, the middle shaft 12 and the drive shaft 13 which are disposed to confront each other and are coupled together, two output shaft bearing portions (radial ball bearings 41, 42) provided on the first and second casing members 31, 32, respectively, for rotatably supporting the output shaft 11, two middle, shaft bearing portions (radial ball bearings 43, 44) provided on the first and second casing members 31, 32, respectively, for rotatably supporting the middle shaft 12, and two drive shaft bearing portions (radial ball bearings 45, 46) provided on the first and second casing members 31, 32, respectively, for rotatably supporting the drive shaft 13.
According to the power transmission mechanism for a front and rear-wheel drive vehicle, the respective output shaft, middle shaft and drive shaft are rotatably supported by the respective bearing portions on the two coupled casing members and are accommodated in the two casing members. Consequently, in assembling the power transmission mechanism, not only can the respective rotating shafts are supported on the bearing portions on the respective casing members by providing the respective bearing portions at the predetermined positions on the casing members but also the three rotating shafts can be positioned relative to one another with ease and accuracy only by coupling the two casing members.